HVAC systems and components, i.e. systems and components in the field of heating, ventilation and air conditioning exist in many different designs. One of them are systems and components which are designed to nominally work in a given temperature range but are also equipped with specific features which allow them to work in exceptional situations under substantially different temperature conditions too, at least for a certain period of time. Such exceptional situations include for example fire and smoke situations.
One of the tasks performed by some HVAC components is to regulate the flow of a fluid such as a water or air flow through a flow channel such as a tube, an air duct, a channel or the like. This is often accomplished with dampers or valves which are arranged in or near the flow channel and which are controllable to open or close the flow channel to a given extent.
Electric motors are often used to regulate the position of the damper or valve in order to achieve a desired rate of flow of the fluid in the flow channel. A disadvantage of electric motors is however that the outpour power of the motor decreases with an increasing temperature. One of the reasons for this decrease is the fact that the current source is not ideal. Another reason is that the magnetic field in an electric motor decreases because the magnet becomes weaker. Accordingly, in the case of an increased ambient temperature the motor torque decreases which may result in a malfunction of the entire system since the decreased motor torque does no longer suffice to actuate the damper or valve.
In a known solution of this problem, the electric motors and therewith also the motor control and other units of the system are dimensioned to fulfill the torque requirement at the higher temperatures. Accordingly, the size of the motor has been chosen such that the reduced motor torque at the higher temperatures suffices to actuate the damper or valve. Or in other words, the motor as well as other components have been substantially oversized resulting in higher space requirements and increased costs not only for the motor but also for other system components.
Another HVAC actuator is disclosed in US 2010/0123421 A1 from Honeywell. The actuator includes a controller that controls the output torque of a motor in response to various sensor signals such as position or speed sensors.
Whereas such a digital torque control would generally be possible to control the torque in case of an increasing ambient temperature, it shows increased part and space and therewith also cost requirements, since additional components such as sensors or a complex motor controller are needed for its implementation.
EP 0 895 346 A2 also from Honeywell discloses a further HVAC actuator being a two-position spring return actuator and that is able to properly work at an elevated temperature. The actuator includes a temperature sensor and a PWM (pulse width modulation) controller that controls the voltage at which current is supplied to the motor. In particular, the controller controls the voltage at which current is supplied to the motor in dependency of the sensed temperature. In order to compensate the decrease of the motor power in case of an elevated temperature, the controller increases the voltage if the sensed temperature exceeds a given limit.
This actuator too requires a complex and therefore expensive control and does not allow for a continuous temperature dependent voltage control but only two temperature input values, either below or above a given limit.